1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, in order to supply electric power to an electronic device, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) has begun to come into commonplace use. In order to advance the compatibility of products between manufacturers, the WPC (Wireless Power Consortium) has been organized, and the WPC has developed the Qi standard as an international standard.
FIG. 1 is a diagram showing a configuration of a wireless power supply system 100 that conforms to the Qi standard. The power supply system 100 includes a power transmitter 200 (TX) and a power receiver 300 (RX). The power receiver 300 is mounted on an electronic device, examples of which include cellular phone terminals, smartphones, audio players, game machines, and tablet terminals.
The power transmitter 200 includes a transmission coil (primary coil) 202, a driver 204, a controller 206, and a demodulator 208. The driver 204 includes an H-bridge circuit (full-bridge) circuit or otherwise a half-bridge circuit. The driver 204 applies a driving signal S1, specifically, a pulse signal to the transmission coil 202, such that a driving current flows through the transmission coil 202, thereby allowing the transmission coil 202 to generate an electric power signal S2 in the form of an electromagnetic field signal. The controller 206 integrally controls the overall operation of the power transmitter 200. Specifically, the controller 206 controls the switching frequency of the driver 204 or otherwise the duty ratio of the switching of the driver 204 so as to adjust the electric power to be transmitted.
In the Qi standard, a protocol is defined for communication between the power transmitter 200 and the power receiver 300, which enables information transmission from the power receiver 300 to the power transmitter 200 via a control signal S3. The control signal S3 is transmitted from a reception coil 302 (secondary coil) to the transmission coil 202 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation. The control signal S3 includes electric power control data (which will also be referred to as a “packet”) which indicates an amount of electric power to be supplied to the power receiver 300, and data which indicates the particular information for identifying the power receiver 300. The demodulator 208 demodulates the control signal S3 included in the current or otherwise the voltage applied to the transmission coil 202. The controller 206 controls the driver 204 based on the power control data included in the control signal S3 thus demodulated.
The power receiver 300 includes the reception coil 302, a rectifier circuit 304, a capacitor 306, a modulator 308, a secondary battery 310, a controller 312, and a charger circuit 314. The reception coil 302 receives the electric power signal S2 from the transmission coil 202, and transmits the control signal S3 to the transmission coil 202. The rectifier circuit 304 and the capacitor 306 rectify and smooth a current S4 induced at the reception coil 302 according to the electric power signal S2, thereby converting the current S4 into a DC voltage.
The charger circuit 314 charges the secondary battery 310 using electric power supplied from the power transmitter 200.
The controller 312 monitors the amount of electric power supplied to the power receiver 300, and accordingly generates electric power control data (control error value) which indicates the amount of power transmission. The modulator 308 modulates the control signal S3 including the electric power control data so as to modulate the coil current that flows through the reception coil 302, thereby modulating the coil current and coil voltage applied to the transmission coil 202.
The above is the configuration of the wireless power supply system 100. FIG. 2 is a flowchart showing an operation sequence of the power supply system 100. The state of the power transmitter 200 can be roughly classified into three phases, i.e., a selection phase ϕ1, a power transfer phase ϕ2, and an identification/configuration phase ϕ3.
First, description will be made regarding the power transfer phase ϕ2. The power transmitter 200 (TX) starts power transmission to the power receiver 300 (RX) (S100). The power transmitter TX receives, as a feedback signal from the power receiver RX, the control signal S3 which indicates the present power transmission state (S102). The power transmitter TX adjusts the power transmission rate based on the control signal S3 (S104).
When the power receiver RX transmits the control signal S3 indicating that the charging has been completed (S106) or otherwise when the power transmitter TX detects, according to a communication timeout error control operation, that the power receiver RX has been removed from an area where it can receive the power supply provided by the power transmitter TX (S108), the power transmitter TX stops power transmission. In this stage, the power transmitter TX transits to the selection phase ϕ1.
Next, description will be made regarding the selection phase ϕ1. The power transmitter TX transmits an electric power signal S2 at a predetermined time interval (object detection interval, e.g., 500 msec), so as to detect the presence or absence of the power receiver RX (S200). Such an operation will be referred to as the “analog ping phase” hereafter.
Upon detection of the power receiver RX (S202), the power transmitter TX transits to the identification/configuration phase ϕ3. In this stage, a digital ping phase is executed (S202). In the subsequent identification/configuration phase, the power transmitter TX receives identification information with respect to the power receiver RX (S206). Subsequently, the power transmitter TX receives information with respect to the power transmission conditions from the power receiver RX (S208), following which the power transmitter TX transits to the power transfer phase ϕ2. The above is the operation sequence of the power transmitter 200.
As a result of investigating such a power supply system 100, the present inventors have come to recognize the following problem.
The charger circuit 314 is switchable between a constant current (CC) charging mode and a constant voltage (CV) charging mode according to the state of the secondary battery 310. In the CC charging mode, the charger circuit 314 adjusts the value of the charging current supplied to the secondary battery 310.
FIG. 3 is an operation waveform diagram showing the operation of the power receiver 300 shown in FIG. 1. In a steady state, such an arrangement provides a balance between the current supplied from the rectifier circuit 304 to the capacitor 306 and the current supplied from the capacitor 306 to the charger circuit 314, i.e., the charging current Ibat. In this state, the rectified voltage Vrect that develops across the capacitor 306 is stabilized to a target level.
With such an arrangement, the current supplied from the rectifier circuit 304 to the capacitor 306 corresponds to the electric power supplied from the power transmitter 200 to the power receiver 300. That is to say, the current supplied from the rectifier circuit 304 to the capacitor 306 is controlled according to the control signal S3. If the charger circuit 314 increases the charging current Ibat, a large current is drawn from the capacitor 306. This reduces the rectified voltage Vrect, leading to an increased control error value included in the control signal S3. In this state, the power transmitter 200 is subjected to a feedback control operation so as to raise the power supply to the power receiver 300. The speed of such a feedback control operation is limited by the communication rate of the control signal S3 and the time required for the power transmitter 200 to stabilize to a new operation point. Thus, if the charging current Ibat suddenly changes, in some cases, the feedback control operation does not follow such a sudden change. In some cases, this leads to marked deviation of the rectified voltage Vrect from its target value. If there is a large change in the rectified voltage Vrect, or if the rectified voltage has a sharp change in its waveform, this leads to adverse effects on the AM modulation of the control signal S3 employing backscatter modulation, resulting in a problem in that the power transmitter 200 cannot receive the control error value in a normal manner. That is to say, in some cases, such a sudden change in the charging current Ibat leads to cutoff of the feedback loop. If the disconnection of communication between the power transmitter 200 and the power receiver 300 continues for a predetermined timeout period, the power transmitter TX stops power transmission, and returns to the selection phase ϕ1.
It should be noted that such a problem is by no means within the scope of common and general knowledge of those skilled in this art.